1. Field of the Invention
This invention relates to a combustor of a gas turbine, which is preferably applied to a gas turbine having a high pressure ratio, and which can be designed to achieve stable combustion and reduce the occurrence of nitrogen oxides (NOx)
2. Description of the Related Art
A gas turbine used for power generation is composed of a compressor, a combustor and a turbine as main members. Many gas turbines have a plurality of combustors, mix air compressed by the compressor and a fuel supplied to the combustors, and burn the mixture in each combustor to generate a combustion gas at a high temperature. The high-temperature combustion gas is supplied to the turbine to drive the turbine rotationally.
An example of the combustor of a conventional gas turbine will be described with reference to FIG. 8.
As shown in FIG. 8, a plurality of combustors 10 of the gas turbine are arranged in an annular configuration in a combustor casing 11 (only one combustor is shown in FIG. 8). The combustor casing 11 and a gas turbine casing 12 are full of compressed air and constitute a casing 13. Air compressed by a compressor is introduced into the casing 13. The introduced compressed air enters the interior of the combustor 10 through an air flow inlet 14 provided at an upstream portion of the combustor 10. Inside an inner tube 15 of the combustor 10, a fuel supplied through fuel nozzles 16 and compressed air are mixed to burn. A combustion gas produced by combustion is supplied to a turbine room through a transition pipe 17 to rotate a turbine rotor.
In recent years, environmental regulations have been tightened, and various improvements have been made to decrease the concentrations of NOx (nitrogen oxides) in an exhaust gas from the gas turbine. The so-called rich-lean combustion process is known as a gas turbine combustion technology designed to reduce the occurrence of NOx.
According to the rich-lean combustion process, combustion in a fuel-rich state (i.e., rich state) is performed in a first combustion region (in the example of FIG. 8, for instance, a middle region of an internal space of the inner tube 15), while combustion in a fuel-lean state (i.e., lean state) is performed in a second combustion region (in the example of FIG. 8, for instance, a peripheral edge region of the internal space of the inner tube 15). In this case, the fuel-air ratio of the entire combustor (i.e., the overall fuel-air ratio obtained by averaging the fuel-air ratios in the rich state and the fuel-air ratios in the lean state) is controlled so as to take a value corresponding to the operating state of the gas turbine. In the case of such rich-lean combustion, complete diffusive combustion (combustion in the absence of premixing with air) takes place in the rich combustion region, and complete premixed combustion is performed in the lean combustion region.
As is generally known, the relationship between the fuel-air ratio (equivalence ratio) and the amount of NOx generated is as shown in FIG. 9. A large amount of NOx is generated at an equivalence ratio φ in the vicinity of 1, and the amount of NOx generated is small in the lean region (φ<1) and in the rich region (φ>1). The rich-lean combustion process can reduce the amount of NOx generated, as a whole, because this process enables combustion to be performed in the lean region (φ<1) with a small amount of NOx generated and in the rich region (φ>1) with a similarly small amount of NOx generated.
For reference, the equivalence ratio will be described.
The equivalence ratio φ is defined as the ratio mf/mf* where mf* is the amount of fuel which uses 100% of oxygen in a certain amount of air and completely burns, and mf is the amount of fuel which is actually given.
Based on φ=1, φ<1 is taken as fuel-lean (lean) and φ>1 as fuel-rich (rich).
The inventor of this invention is developing a 1700° C.-class gas turbine at a high pressure ratio (pressure ratio of 25 or higher). With such a gas turbine at a high pressure ratio, the temperature of air flowing into the combustor is very high (500 to 600° C.). With a conventional gas turbine, the pressure ratio was of the order of 20 to 21, and the temperature of air flowing into the combustor was of the order of 450° C.
The gas turbine at such a high pressure ratio has been found to pose the following disadvantages, if the rich-lean combustion technology of the prior art is employed unchanged:    (1) Since the pressure ratio is high and the air (gas) energy is high, flashback is very likely to occur at a premixing nozzle portion for forming lean combustion.    (2) On the rich combustion side, complete diffusive combustion at an equivalence ratio of 1 takes place. Thus, the temperature of air flowing into the combustor is very high (500 to 600° C.). Partly because of this, the temperature of the flame front becomes so high that NOx is prone to occur.
The present invention has been accomplished in light of the above-described problems with the earlier technologies. It is an object of the invention to provide a combustor which can achieve stable combustion and decrease NOx even in a gas turbine having a high pressure ratio now under development.